Waters flow east-to-west into the North Sea through the Skagerrak Strait, an outlet to the Atlantic Ocean from the Baltic Sea.

Along the southern reaches of the North Sea, waters from the Atlantic Ocean flow through the English Channel and on into the North Sea through the Strait of Dover.

The Kiel Canal in northern Germany, is one of the world's busiest artificial waterways. International sea traffic uses it to connect between the North Sea and the Baltic Sea, thus saving hundreds of miles of additional travel time around Denmark, and the related high costs of transportation.

In the southern areas the North Sea's depth is less than 150 ft (50 m), but in the north it slopes down to about 600 ft (200 m).

Along the southwestern coastline of Norway, the North Sea reaches a depth of near 1,200 ft (400 m).